In such stations the primary radar has the function of detecting the presence of an aircraft in the sky and providing information thereon, such as its distance, azimuth and/or elevation and optionally speed, according to the antenna type used.
The secondary radar associated with the primary radar in the station makes it possible to obtain identification information on this aircraft and possibly details of its altitude when that object is equipped with a transponder designed for this purpose.
The aerial of such a radar station comprises an antenna for the primary radar and one or two antennas for the secondary radar. Existing secondary radars must be able to transmit and receive two different signals. The first is called the interrogation signal and is transmitted and received by a corresponding antenna, whereas the second is called the control signal and serves to inhibit all interrogations made in directions other than that of the major lobe of the radiation diagram or pattern of the interrogation antenna.
Thus, there is an antenna for transmitting and receiving interrogation signals and this antenna has a radiation diagram with a major lobe and minor lobes. The control signals are transmitted and received either by another antenna or by the same interrogation antenna. The radiation diagram of the control channel can be of omnidirectional or differential type in the azimuthal plane. In all these cases the control diagram covers the interrogation diagram, except for a small area which is centered on the major interrogation lobe and is called the interrogation arc.
An air-traffic-monitoring station incorporating a secondary radar whose aerial is in accordance with what has been stated hereinbefore, i.e. radiating a diagram of the difference type in azimuth, has been disclosed in commonly owned U.S. Pat. No. 3,916,414 and functions in a manner considered to be satisfactory. However, the azimuthal resolution of such a radar, i.e. its capacity to distinguish between two aircraft which are relatively close to one another, is not very fine. This is liable to cause problems to the radar operators in the station in connection with the identification of interrogated aircraft or targets, mainly at close range. This lack of resolution is mainly due to the width of the interrogation arc, which is inherent in the system and difficult to influence.
Therefore this inadequate azimuthal resolution causes problems in connection with the identification of interrogated targets, particularly when the latter are crossing, in parallel paths or on waiting lines.
One presently adopted solution consists in having prohibitive spacing requirements which are prejudicial to the traffic flow, this being rather impractical in view of the fact that radar stations are having to deal with an ever-increasing number of aircraft.